SOME PROPERTIES OF THE ECDYSTEROID RECEPTOR IN THE SALIVARY-GLAND OF THE IXODID TICK, AMBLYOMMA-HEBRAEUM

Salivary gland degeneration in ixodid ticks is triggered by an ecdyste roid hormone. We used [H-3]ponasterone A (PoA) as a specific ligand to detect the ecdysteroid receptor in the salivary glands of large, part ially fed female ticks (Amblyomma hebraeum Koch; Acari: Ixodidae). Bin ding of [H-3]PoA was thermolabile and sensitive to pronase, but not to DNase or RNase, indicating that the ligand binds to a protein. Scatch ard analysis of [H-3]PoA binding strongly suggested the presence of an ecdysteroid receptor in cytosolic and nuclear extracts of the tissue. The K-d and B-max for PoA binding in cytosol were 0.72 +/- 0.09 nM an d 175 +/- 12 fmol/mg protein, respectively (n = 8). Corresponding figu res for nuclear extract were 1.1 +/- 0.5 nM and 282 +/- 35 fmol/mg pro tein, respectively (n = 3; P > 0.05 compared to cytosol). The relative ability of unlabeled ecdysteroids to compete for [H-3]PoA binding was (in descending order): PoA > muristerone A > makisterone A > 20-hydro xyecdysone > mesylinokosterone > ecdysone. The K-d estimated for 20-hy droxyecdysone (probably the natural hormone) correlates very well with its physiological potency in inducing salivary gland degeneration in vivo and in organ culture. None of the vertebrate steroids tested (est radiol, testosterone, progesterone, and corticosterone) was able to di splace PoA binding at a concentration 10(5) times higher than PoA. The cytosolic form of the receptor migrated to the 3.2 S region of a 10-4 0% sucrose density gradient. (C) 1995 Academic Press, Inc.